After a spate of damaging incidents in which donors backed out of high-profile gifts, many colleges, cultural centers and other nonprofits are making it tougher for philanthropists to renege on pledges. A number of institutions, including the San Francisco Symphony, are asking donors to sign legally binding gift agreements rather than rely on the looser arrangements some used in the past. Others, such as New York University, are keeping better tabs on pledges, sending frequent reminders or calling givers to keep better track of money coming in. Some are investigating the finances of potential givers to ascertain their ability to complete pledges.